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Primarily H1-receptor antagonists, this class reduce the inflammatory and allergic reactions such as rhinitis, insect bites, drug allergies and urticaria. They act by reversible competitive antagonism, thus inhibiting histamine-induced smooth muscle contraction. This class possess many side effects (some therapeutic and some undesirable) due to the similarity of the antihistamine chemical structures to the structure of drugs that affect a-adrenoceptor, serotonin, muscarininc cholinoceptor and local anaesthetic receptor sites. These effects include sedation, antinauseaantiemetic action, antiparkinsonian action, blurred vision, orthostatic hypotension and local anaesthesia. Examples include pheniramine, chlorpheniramine, promethazine.
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